Gravity torsion balance



F. E. WRIGHT E AL Oct. 15, 1940.

GRAVITY TORSION BALANCE- Filed Dec. 15, 1938 6 Sheets-$heet l KY 1 ii xx Oct. 15 1940.

F. E. WRIGHT ET AL GRAVITY TORSION BALANCE Filed Dec. 15. 1938 6 Sheets-Sheet 3 A A A A Oct. 15, 1940. F. E. WRIGHT ET Lin.

GRAVITY TORSION BALANCE Filed Dec. 15, 1938 s Sheetsheet 4' Oct. 15, 1940. F. E. WRIGHT El AL 2,218,140

GRAVITY TORSION BALANCE Filed Dec. 15, 1938 6 Sheets-Sheet 5 Oct. 15, 1940.

F. E. WRIGHT ET AL GRAVITY TORSION BALANCE Filed Dec. 15, 1938 6 Sheets-Sheet 6 l I I I l I I I I I l I l I l I l I I I I Patented Oct. 15, 1940 UNITED STATES GRAVITY TORSION BALANCE Frederick E.

Wright and Joseph L. England,

Washington, D. 0., assignors to Carnegie Institution of Washington, Washington, D. (7., a corporation of United States Application December 15, 1938, Serial No. 245,994

9 Claims.

This invention relates to gravity torsion balances or apparatus for the measurement of the variations in the force of gravity.

The invention relates to a torsion balance of the type described and claimed in Patent No. 1,579,273, granted April 6, 1926 to Frederick Eugene Wright, one of the present inventors. The essential elements of such a torsion balance comprise a helix of wire mounted in a frame or cradle that is rotatable about a horizontal axis passing through the ends of the helix, a radial boom or weight arm secured to the center of the helix, and devices for measuring the angular displacements of the frame which result in a rotation f the radial boom into horizontal positions at opposite sides of the helix. The elastic deformation of the helix, by rotation of the frame to raise the boom from its normal position depending below the helix, establishes a stress in the helix which counterbalances the force .of

gravity. Reference is made to the prior patent for the method of operation of apparatus of this type.

An object of the present invention is to provide a gravity torsion balance of the type stated in which the various parts associated with the helical spring and its rotatable frame are of new physical design and contribute to the stability of the apparatus. An object is to provide a torsion balance of the horizontally arranged helix and radial boom type, the balance including movable members for supporting the helix and look-- ing the boom against movement when the apparatus is not in use. A further object is to 5 provide a torsion balance of the type stated in which all moving parts are in a hermetically sealedcylinder from which the air is exhausted.

Other objects relate to the provision of improved forms of auxiliary devices such as illuminating systems, levels, temperature indicating systems, and mechanisms for rotating the frame that carries the helix. Still another object is to provide a gravity torsion balance including a therr many-responsive connection between the helical spring and its supporting frame, the arrangement being such that temperature changes in the connection compensate partially for changes in the rigidity modulus of the helical-springwith 50 changes in temperature.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings in which: v

55 Fig. 1 is a fragmentary side elevation, with parts in section, of an embodiment of the invention as set up in operative position within Ia truck;

- Fig. 2 is a fragmentary plan view of the apparatus shown in Fig. 1; 5 Fig. 3 is a central vertical section, on a larger scale, through the torsion balance and its heat insulated housing;

Fig. 4 is an end elevation of the same;

Fig. 5 is a fragmentary central vertical section, on a larger scalethan Fig. 3, of the spring helix and the supporting pans in which it rests when the apparatus is not in use;

Fig. 6 is a fragmentary horizontal section as seen on line 66 of Fig. 5;

r Fig. 7 is an enlarged plan view of the meeting ends of the supporting pans;

Fig. 8 is an enlarged end elevation of the torsion balance casing as seen when removed from the outer housing; 20

Fig. 9 is a transverse section of the instrument case, as seen from the left of Fig. 3, after, re; moval of the casing cap;

Fig. 10 is a fragmentary end elevation, with parts in section, of the fine adiusting mechanism for rotating the frame;

Fig. 11 is a transverse section substantally on line ll-H of Fig. 10;

Fig. 12- is a fragmentary transverse section substantially on line l2-l2 of Fig. 4; and 30 Figs. 13 and 14 are fragmentary transverse sections of the helix, its supporting frame and temperature compensating devices for connecting the helix to the frame. a

In Figs. 1 and 2, the reference numeral l identifies the floor of a "truck in which the apparatus may be transported or set up, from the. ground below the truck, for the taking of measurements. The outer cylindrical shell 2 of the protective housing of the apparatus rests in and 40 is secured to a cradle 3 by clamp rods 4 that are drawn down on shock absorbing strips 5 of sponge rubber or the like that rest on the upper half of the housing.

When set up, for use, as shown in solid lines, the cradle 3 rests on leveling screws 6 that extend through a table I that has telescoping tripod legs 8 that are extended to rest on the ground beneath the truck. The legs extend through T- shaped openings 5 in the floor l and have cross pins III that may be engaged behind pins II on the floor, by a partial rotation of the table, when raised, to support the table from the truck floor. =A spring I2 is connected between the bottom-of 5 the table and the floor of the truck to hold the table in stable raised position during transportation. The cradle 3 and instrument housing 2 are transferred to a platform l3 when the apparatus is not in use. Platform l3 rests on a resilient support, specifically an automobile inner tube I4 that is held in place on the floor by angle brackets l5, and the platform is resiliently held against movement by a plurality of springs IS.

The instrument and its housing 2 are quite heavy and an I-beam I1 is secured to the top of the truck to facilitate movement of the apparatus by a hoist l8 on the trolley l9 which rolls along the lower flanges of the beam l'l. When moved to inoperative position, as shown in dotted lines, the casing may be restrained from vibration by a plurality of springs 20 that are, connected between the housing and the truck.;

The protective housing'has aitliick layer of heat insulating material 29, see Fig. 3, between the outer shell 2 and the comp tments formed, respectively, by a rear shell :l'in which the instrument is located and a forward drum 22 through which a plurality of tubular passages are formed by walls 23. Both compartments open through the top wall of the shell 2 for receiving a medium, indicated by the legend Refrigerant An annular plate 21 is secured to the forwardend of the shell II and to the correspondingly arranged end flange or the tubular container 28 which receives the case of the torsion spring mechanism with a close sliding fit. The rear end of the container 28 is supported axially of the shell 2| by a plate 2! of insulating material that is perforated 6r notched, as shown, to permit ice and/or water to pass to the rear of the inner chamber.

The various shells and walls of the housing are preferably formed of stainless steel. Material of this type is desirable throughout the housing and particularly so for the chamber 28 to preclude electrolytic action between it and the instrument case which is of cast aluminum alloy.

The case comprises a forward tubular section 3i, and an end cap flthat is hermetically fitted upon the rear end of the tubular section. A yoke 33 secured to the rear end of case section SI is axially bored to receive the tubular hub 34, see Fig. 5, that is fixedto the rear bar 35 of the frame by which the helix 38 is supported.

Side members 81. II of the frame are rigidly secured between the rear bar I! and the front bar 3!. A tubular shaft 4| is secured to the front bar and journalled in the axial bore of the transverse wall 3| of the case section 30. The flanged collar ll of a slow .motion drive, to be described later with reference to Figs. 10 and 11, is keyed to the shaft 4.. The cupped end of a coupling member 43 is secured between the collar 4| and the outer tubular shaft 44 which extends through the front end of the housing and carries a knob l5 that may be turned manually to rotate the frame.

The ends of the helix ll of tungsten wire are secured in axial bores in small rods 48 that are mounted in a bracket 41 on the forward frame bar 39 and in the tubular hub 34 of the rear bar 35. The mountings of the rods 46 are similar but only the rear mounting is shown in detail in 5 'to clamp the gland 48 upon the rod 48 and to the bushing. 10

A boom or radial arm 5| is secured to the center of the helix 36 and carries a transverse wire 52 that has a signal 53 at each end. These signals are fine glass filaments that extend parallel to the axis of rotation of the helix and 1 its frame. The advantage of the glass filament type of signal is that, when viewed by transmitted light, it has the appearance of a narrow bright band between two black bars. The diameter of the filament is such that the light band is but 20 slightly wider than the crosswire of the telescope with which it is to be viewed, and the light band may therefore be accurately superposed on the crossbar. I

As shown in Fig. 3, and in greater detail in 25 Figs. 5 to 7, mechanism is provided for supporting the helix in unstressed condition when the apparatus is not in use. This mechanism comprises a pair of shallow troughs or pans 54 that arepivoted to the end barsfi, 39 of the rotat- 80 able frame. The inner ends of the pans overlap and are slotted, see Fig. '1, to clear the boom ll. Links 55 are pivoted to the pans and to links I which terminate in gear segments 51 in mesh with worms 5. on a shaft 5! that extends along 35 the side frame 31 of the frame. A cross shaft ll and bevel gears connect the shaft 59 to the ad- Justing shaft I that lies within the hollow shaft 40 of the'frame. The outer end of shaft 81 has a kerf for receiving a screw driver that may be 40 inserted through the outer hollow shaft 44.

The boom ii is resiliently held against movement by s-shaped springs 62 of-fiat phosphor bronze on the ends of arms '3 that terminate in yokes 64 which are pivoted to the links I. u The lower outer edges of the arms 83 are cut away to allow the arms to fold down, as shown in dotted line, when engaged by the pans as they move to lowered position. The'upper edge of each bar 63 engages its link I to limit upward movement of the arms by springs 65 that are, connected between the yokes l and the associ- .ated gear segments 51. The worms 5! are threaded in opposite sense and the links it therefore rock in opposite directions when the shafts are rotated. The manner in which the pans I4 and clamp arms 63 move into inoperative position will be apparent from inspection of Fig. 5.

A scale plate or graduated circle 8' is secured to the hub of the frame bar 39, see Figs. 3 and 8, to cooperate with a vernier plate 61 m that is secured to the case section II in line with a hermetically sealed window 68 in 'the transverse wall. A telescope 69 for viewing the vernier plate and adjacent section of the scale plate is mounted on an arm II that is journalled on the hub or wall ll for limited angular movement along the vernier plate 61. The scales are illuminated by light transmitted from a small electric bulb II, in a casing 12 secured to the telescope, through a clear glass rod 13 of Pyrex" glass or quartz that has its inner end. bent towards the window I.

The telescope I and lighting system pass through arcuate slots 13' in the cover 14 that closes the forward end of thecase section I! and through one of the longitudinal passages through the forward compartment 22. The slots 13' are substantially closed against the entrance of air by plate I that is journalled on the shaft 44, the telescope 89 and glass rod 13 having a snug fit where they pass through the plate.

Telescopes 18R, 18L for the primary observation of the signals 53 when the boom is raised are secured to right and left walls, respectively, of the case section 38 and extend to the front of the housing through openings in the cover I4 and passages through the drum compartment 22. Prisms 'IIR, "L are mounted at the inner end of case section 38 in line with the respective telescopes to direct the line of sight inwardly towards positions into which the signals 53 are raised by rotation of the fram and helix. The signals are illuminated by a lamp bulb 18 in a housing I9, Fig. '12, that is fitted over one of the telescopes 18R, 18L when the observation is to be made with the other.

These telescopes also serve to read the temperature within the instrument case from thermometers 88 that are mounted on the frarne members 31, 38 with their bulbs at opposite ends of the frame. Both thermometers are read by the telescope 18R with the arrangement shown in Fig. 9, and both are illuminated when in reading position by light transmitted through the telescope 18L. 'The telescope 18B, is normally focused on the point a which the glass filament 53 reaches when the boomis elevated. The thermometers will be at point D on the line joining the prisms 11R, 11L when temperature readings are to be made. It is possible to refocus the telescope but this is not necessary with the apparatus shown in Fig. 9. Negative lenses 8| are mounted in each frame member in such positions, as indicated by thebroken lines, as to lie in front of the prism 'I'IR when the thermometer on the opposite frame member is in line with the prism TIL. The negative lens increases the focal length of the objective of the telescope 18R to bring it in focus at the point b.

The Vernier or slow motion mechanism for making the final adjustment of the frame is shown in Figs. 8, I0 and 11. The upper annular end of a rock arm 8| fits loosely about the collar 4| and is held in place by the annular plate 83. Plate 83 has been removed, as the parts are shown in Fig. 10, to show the locking block 84 that is loosely seated in a recess in the arm 8| in line with the push rod 85 that rests on the conical section 88 of the cam rod 81. Rod 8! is threaded into a bushing 88 and, when moved to the right, Fig. 11, forces rod 85 upward to clamp the block 84 to the collar 4|. The lower end of rod 85 is cut at an angle and curved to fit the cam 88, the rod being held against rotation by the guide pin 89 that engages a slot in the rod. Cam rod 81 extends to the front of the housing and is fitted with a handle 98, Fig. 8.

The lower end of arm 8| is provided with a lug 9| that extends between the spring'pressed plunger 92 and the adjusting screw 93. The plunger and screw are in axial alinement in the block 94 that is secured to the case section 38. The outer end of the adjusting screw slides in the hub of a bevel gear 95, the gear being arranged in a slot in the block 94 and keyed to the adjusting screw. A section 98 of the threaded bore for the screw 93 is removable, see Fig. 8, to facilitate the assembly of the screw and gear 95 on the block 94. The hollow adjusting shaft 91 has a bevel gear 98 in mesh with gear 95 and is provided with an adjusting knob 99, Fig. 4.

A pair of Zeiss levels 99L,.99T are mounted on a bracket I88, at the forward end of case section 38, with their axes parallel to the axis of helix 38 and to a horizontal line transverse to that axis. .The levels include prism systems I8IL, I8IT, respectively, that permit a reading of the level from a point in a plan through the axis of the level. Additional prisms, I82L, I82T, are mounted on a post I83 in line with the Zeiss level prisms and so disposed that images of the two levels are formed in vertical alinement. A flared tube I84 is secured to the cover I4 in line with the prisms I82L, |82T and. the telescope I85, and the levels are illuminated by a lamp bulb in the casing I88 and the clear glass rod I81.

A pipe I88 is threaded into the transverse wall 3| of the case section 38 and terminates in a T-coupling at the front end of the housing. A flexible tube I89 on one branch of the T extends to a vacuum pump, not showmand flexible tube 8 extends from the other branch to a vacuum gauge III. The pipe H2, shown'at the left side of Fig. 8, serves to supply grease to the bearing of the shaft 48. The outer end' of the pipe is closed by a cap II3, Fig. 4, that may be removed to fit a grease gun to the pipe. The grease serves both as a lubricant and as a seal for the evacuated chamber in which the helix is located.

Temperature effects upon the rigidity modulus of the tungsten helix 38 may be partially compensated by altering the axial length of the helix with changes in temperature.

As shown in Fig. 13, the inner end of the helix 38 is carried by the'end of a bimetallic strip 4 which is secured to a block 5 on the frame bar 35. The effective length of the bimetallic. strip may be varied by the slide 8 I the viewing telescopes below the axis of the frame,

the angular spacing of each signal from the boom being such that the upper signal reachfes line ab, Fig. 9, when the boom 5| is horizontal.

For directly comparable results, the readingsat each station should be made in th same order and allu'eadings should be made at a definite time interval after starting the rotation of the frame to raise the boom 5|. Accuracy within one part in a million has been obtained with one embodiment of the invention by rotating the frame and helix, as uniformly as is possible in manual operation, to bring the signal at the proper level at one side of the axis for a reading exactly three minutes after the rotation of the frame is started, then returning the frame at a more rapid rate to bring the boom 5| to vertical position and, exactly one minute'after the first reading, starting the three-minute rotation of the frame to obtain: the reading with the signal at the opposite side of the axis." The one minute intervalpermits the recording of the dition except during the short periods of actual use, substantially eliminate the development of residual strains that would impair the accuracy of'the measurements. The load is applied for only a short time in one direction and the same load is then applied in the opposite direction for the same period. This reversal of the stress counteracts the strains set up by the first stressing of the spring. The spring recovers quickly from any residual strain present at the completion of one set of readings and each measurement is therefore made with the spring in an unstrained condition.

We have described what we now believe to be the preferred embodiment of the invention but it is to be understood that there is some latitude 4 in the design and arrangement of the component parts of the apparatus.

We claim:

1. In a torsion gravity balance, the combination with a horizontally arranged helix of elastic material, a frame carrying the helix, means including a hollow shaft supporting said frame for rotation about the axis of the helix, a boom extending radially from the center of the helix, and means for indicating the angular rotation of the frame which displaces the boom into horizontal positions at opposite sides of the helix,

.of means for supporting the helix when the apparatus is not in use, said supporting means comprising elongated pans pivoted to the ends of the frame and positioned beneath the helix, a linkage system for raising and lowering each pan, and operating means for said linkage systems extending into said hollow shaft.

2.- The invention as claimed in claim 1, in combination with means carried by said linkage systems for resiliently holding said boom against displacement when the pans are in raised position to support the helix.

3. In a torsion gravity balance, the combination with an evacuated case; a frame pivotally supported in said case for rotation about a horizontal axis, a helix of elastic material carried by said frame, a boom secured to said helix at the center thereof, an arm transversely secured to said boom in a plane at an angle to the axis of said helix, and signals on said arm and equally spaced from the boom, of optical systems for viewing the lower signal when the frame is rotated to displace the boom to opposite sides of the helix, said optical systems including a telescope at each side of .the case below the axis thereof; and a prism at each side-of the case for directing the line of sight of each telmcope transversely of the case and towards the other prism,

whereby light directed through one telescope illuminates the signals as the boom is raised towards the other telescope.

4. The invention as claimed in claim 3 wherein said signals comprise fine glass filaments.

5. A torsiongravity balance comprising an evacuated case, a helix of elastic material and a frame supporting the same in the case for rotation about its axis, a radial boom secured to said helix, and means for indicating the angular displacements of the frame which raise the boom to horizontal position at opposite sides of the helix; said indication means including a graduated circle and a vernier plate within the case, and a window hermetically sealed to the case for viewing the graduated circle and vernier plate.

6. In a torsion gravity balance of the type including an evacuated case housing a pivotally mounted frame carrying a helix from which a boom depends, and a pair of telescopes and prisms secured to opposite sides of the case and in line with each other below the axis of the frame for observing signals carried by the boom means for observing the temperature within the case,

said means comprising a thermometer carried by the frame and having a stem in the field of view of one telescope, an opening in the frame in line with the thermometer stem, a negative lens in an opening in the opposite side of the frame for alining with one prism when the frame is rotated to position the thermometer stem in line with the other prism, said negative lens increasing the normal focal length of the associated telescope to focus on the thermometer stem.

7. In a torsion gravity balance, an evacuate-d case housing a rotatable frame carrying a helix to which a boom is secured, a shaft secured to said frame and extending to the exterior of said case, a rock arm encircling said shaft, a block on said arm and means for clamping the block to the shaft; said means including a push rod engaging said block, a rotatable cam rod extending parallel to said shaft, said cam rod having a threaded section engaging a correspondingly threaded portion carried by the rock arm and a conical section engaging the end of said push rod, and means for displacing said rock arm angularly to impart a slow motion adjustment to said shaft when the block is clamped thereto.

8. In apparatus of the type described, a case housing a frame, meansat the ends of said case supporting the frame for rotation about a horizontal axis, a pair of levels mounted in the case at right angles to each other,.the axes of said levels being -parallel,- respectively to the axis of said frame and to afliorizontal line transverse to the axis of said frame, prisms associated with each level to permit viewing the same from the end of the level auxiliary prisms adjacent each other for viewing the two prism systems from an end of said case, a cover closing said case, a telescope extending to the exterior of said case for viewing the auxiliary prisms, a clear glass rod extending into said case, and a light bulb at the exterior of said case in line with said rod. for ilin said drum compartment, a housing enclosing said compartments, and heat insulating materi"l between said housing and said compartments, each compartmenthaving an opening extending to the exterior of said housing at the top thereof.

. FREDERICK E. WRIGHT. JOSEPH L. ENGLAND, 

